The present invention relates generally to monaural signal to artificial stereo signals converting and processing circuits for headphones, and more particularly to a circuit capable of converting and processing a monaural signal into artificial stereo signals, and supplying the signal thus obtained to a headphone, so that the sound expansion felt by a listener extends into the region outside the listener's head.
Generally, so-called binaural system is known, in which microphones are provided at the positions of the two ears of a dummy-head having a shape of a human head, the sounds received at the positions of the two ears are respectively recorded, and the sounds thus recorded are reproduced and respectively supplied to both speakers of the headphone. By this system, the listener can hear these sounds as though the acoustic image were at the same position as that of the actual sound source. In order to obtain this binaural signal, the dummy head must be used.
Accordingly, a signal processing circuit has been proposed in which, a signal substantially equal to the binaural signal, electrically, is obtained from the normal monaural signal or from each of the channel signals of the stereo signals. This signal processing circuit includes two adding amplifiers, and a delay circuit. In the above circuit, the monaural signal not having localizing information is respectively applied to one of the input terminals of the two adding amplifiers. On the other hand, the monaural signal is delayed by the delay circuit and adjusted of its level by a level adjusting device. The signal thus obtained is supplied to one of the adding amplifiers so that the signal undergoes positive phase addition, and also respectively applied to the other adding amplifier so that the signal undergoes reverse phase addition. Hence, the outputs of both adding amplifiers are respectively and separately supplied to the left and right speakers of the headphone.
When the listener holds the left and right speakers respectively at his ears, to listen only from the left speaker intended for the left ear by the left ear and vice versa, the sound expansion is felt as though it extends a little outside the center from within the listener's head. Therefore, a sensation similar to that felt upon simultaneous listening of stereo signals by both ears, is obtained. However, by the conventional circuit, the sound expansion felt by the listener is, at the maximum, felt between the ears, limited within the head, and suffered a disadvantage in that satisfactory stereo effect could not be obtained.